1. Field of the Invention
This invention relates to a process to prepare film and fiber forming synthetic polyesters. More particularly, it relates to the preparation of the film and fiber forming synthetic polyesters of high viscosity either by direct esterification or interchange of an organic dicarboxylic acid and/or the lower alkyl esters thereof with a glycol having 2 to 10 carbon atoms per molecule to obtain the corresponding diglycol-ester intermediate and then condensing said intermediate in a high surface area-to-volume ratio reactor at temperatures between about 275.degree. C. and 300.degree. C., pressures between about 0.1 and 760 Torr, a residence time between about 0.1 and 6 hours to increase the viscosity of the polymer.
2. Description of the Prior Art
The preparation of linear high-molecular weight polyesters useful in commercial articles either by the ester interchange reaction between dimethyl terephthalate and a polyol or by a direct esterification process wherein a dicarboxylic acid or anhydride is reacted with a polyol is known. U.S. Pat. No. 2,465,319 illustrates initial disclosure of the preparation of poly(ethylene terephthalate), and U.S. Pat. Nos. 3,050,533; 3,018,272; 3,484,410; and 3,689,461 illustrate various improvements thereof.
In general, the direct esterification in the prior art is carried out with the molecular ratio of the acid to the polyol of from about 1.0 to about 1.0-2.0 and preferably in a mole ratio of from about 1.0 to about 1.1-1.7.
High intrinsic viscosity (I.V.) polyester is produced commercially by the use of continuous operation reactors designed to create large polymer surface areas by the use of internal wheels, discs, agitators, etc., and in the prior art by the use of high vacuum to strip the product of condensation, e.g., the ethylene glycol. See U.S. Pat. Nos. 3,728,083 and 3,976,431.
The capability of commercial reactors relative to the maximum viscosity and throughput capacity have been governed by the efficiency of film formation, residence time, temperature, and vacuum. The basic construction of the reactor internals determines the amount of film formation and it is thus essentially fixed except by major capital expenditure. Increased residence time and temperature have been used but are detrimental, as known to anyone skilled in the art, due to degradation reactions increasing as residence or temperature is increased. Improved, higher rate catalysts also have been suggested; see U.S. Pat. Nos. 3,701,757 and 3,803,097.
Thus, to increase the maximum intrinsic viscosity capability and/or throughput capacity, polymerization reactors now operate at very high vacuum. Vacuum levels below 1 Torr are common practice, many reactors operate below 0.5 Torr, requiring the use of large expensive vacuum units such as multi-jet steam ejectors. However, it is not generally known to use inert gas sweep or bubbles at such low vacuum levels. U.S. Pat. No. 3,054,776 teaches use of vacuum below 1 Torr during polycondensation, with inert gas bubbled through the melt; however, only 0.1 to 0.4 I.V. polymer is produced in a low surface area reactor.